Method of processing hydrolysed sodium arsenite into commercial product. RU patent 2513846.

FIELD: chemistry.

SUBSTANCE: method of processing hydrolysed technical sodium arsenite into a commercial product involves cyclic repetition of consecutive steps. First, arsenic salts are leached from material with hydrochloric acid solution, which is added until achieving pH 9.5-10.5, to form a heterogeneous system. The heterogeneous system is then divided into a solid phase and a working solution. Further, the working solution is concentrated through evaporation to arsenic (III) content greater than 10 g/100 g water and the concentrated working solution is separated from the formed precipitate. Arsenic (III) oxide is precipitated by acidifying the working solution and the arsenic (III) oxide precipitate is filtered off. The filtrate is returned to the first step of the process. After repeating the cycle of said operations 3 to 10 times, arsenic (IV) compounds are then extracted from the working solution via reduction thereof to arsenic (III) compounds or to elementary arsenic.

EFFECT: invention reduces the amount of process wastes and improves safety when processing hydrolysed sodium arsenite.

2 cl, 2 ex

The invention relates to the field of chemical engineering and may be used in technological scheme of chemical production, the raw material which is arsenite sodium hydrolysis (technical), TU 2622-159-04872702-2005 (hereinafter - ANG). This material has the form of granules from light grey to dark-brown and is a mixture of salts (mainly and arsenite sodium chloride), and small quantities of insoluble in water residue. According to Chapter 5 of the report [1], a number of parties ANG does not meet the technical conditions, in particular, all the investigated party ANG contained salt arsenic (V) - sodium arsenate, in the amount of 2.4% mass to 14.5%mass, the average value of 9,27%of mass. The percentage of arsenic (V) of the total arsenic amounted to 38%of mass.

The objective of the invention is development of a method of ANG processing into marketable products, suitable for processing with possible deviations from the specifications and universal for any non party.

In character composition (mixture of salts) and the limited scale of the task (at present, the reserves of this raw material are approximately 12500 ton), the best is hydrometallurgical technology with selective dissolution of salts arsenic in the first stage and an allocation from a solution oxide arsenic (III) in as the final product. However, the availability of raw materials arsenic (V) complicates the task.

Consider the well-known technologies of processing of arsenic-bearing raw materials, based on hydrometallurgical approach. Known techniques can be classified into 3 groups depending on the obtained product:

1) Oxide arsenic (III)

Method for processing of reaction mass, formed in the detoxification process of lewisite [patent: Demakhin AGI others, 2001 (hereinafter - EN 2192297)].

Method for processing of products of detoxification lewisite [patent: Demakhin A.G. and others, 2001 (hereinafter - EN 2198707)].

Method for processing of reaction mass, formed in the detoxification process of lewisite [patent: Demakhin A.G. and others, 2008 (hereinafter - RU2359725)], and also work Eliseeva A.D. "Physico-chemical bases of process of separation of arsenite sodium hydrolytic on the basic components", Saratov, 2008.

Way food processing alkaline hydrolysis of lewisite in commercial products [patent: Demakhin A.G. and others, 2008(hereinafter - RU2389526)].

2) Technical elemental arsenic

Method of utilization of mixtures containing inorganic arsenic compounds YAP/ [patent: Iwaniec Janusz and others, 2002 (hereinafter - PL 357396)].

The allocation method of elemental arsenic from the reaction of the masses obtained for destruction of lewisite [patent: Baranov SCI and others 2002 (hereinafter - the Russian Federation 2009276)].

The method of obtaining of elemental arsenic from water and water-organic solutions [patent: Seluchenko V.V. and others, 2008 (hereinafter - EN 2371391)].

Method for processing of reaction mass, formed by alkaline hydrolysis of lewisite, technical products [patent: Rastegaev O.YU. and others, 2009 (hereinafter - EN 2396099)].

The method of obtaining of elemental arsenic [patent: Rastegaev O.YU. and others, 2008 (hereinafter - EN 2409687)].

The method of obtaining of elemental arsenic and sodium chloride of the products of alkaline hydrolysis of lewisite [patent: Demakhin A.G. and others, 2009(hereinafter - EN 2412734)].

3) Other products

Method for processing of reaction mass detoxification lewisite [patent: Petrov, V.G. and others, 1995 (hereinafter - the Russian Federation 2099116)].

The method of disposal of toxic substances skin blister agent action type lewisite [patent: Gormy V.V. and others, 1999 (hereinafter - RF 2172196)].

Consider the advantages and disadvantages of technologies referred to in these patents.

Processing technology of arsenic-bearing raw materials in technical oxide arsenic (III)

All of the above technologies associated with obtaining technical oxide arsenic (III)relate to the processing of other kinds of raw material - liquid reaction mass from the lewisite destruction, relevant TU 2112-123-04872702-2002 (hereinafter - liquid reaction mass). In addition to the different aggregative States, a significant difference between the raw materials and the ANT is the high content in APG compounds of pentavalent arsenic.

The technologies described in the patents EN 2192297, EN 2198707, describes how to get oxide arsenic (III) concentration and acidification liquid reaction of the masses, but does not address the problem of withdrawal of arsenic (V) of the workflow, so you can conclude that up to 38% of the arsenic contained in raw materials, will be in the production waste in case of application of these technologies for processing ANG.

Technology EN 2359725 offers oxide arsenic (III) a similar way as in the previous two patents, but includes the method of restoration of arsenic (V) arsenic (III). The method involves the extensive use of chloride, arsenic (III), formed by acidification of the solution of liquid reaction mass with hydrochloric acid to a pH of less than 1.

Chloride, arsenic (III) is volatile (t Kip =130 C), a highly toxic compound (LD 50 =48 mg/kg), and is included in List 2 of the precursors of chemical weapons, established by the Convention on the prohibition of chemical weapons [2]. This matter requires the highest standards of industrial and environmental safety and greatly increases the cost of such a technical process.

In addition, the liquid reaction mass used in the technology as raw materials contain products of alkaline hydrolysis of lewisite reactions

Accordingly, the processing of raw materials hydrochloric acid on the technology in question is made up of phase separation of insoluble organic impurities from a solution of salts arsenic, severe acidification of reaction mass can lead to occurrence of a reverse process:

The reaction (6) is a classic response of receipt of lewisite [3], as a catalyst - acid Lewis is excessive chloride arsenic. Thus, described in RU2359725 the process is reverse to alkaline hydrolysis used for destruction of lewisite, and can lead to the re-education of chemical weapons.

In technology EN 2389526 considers the restoration of arsenic (V) arsenic (III) if using multi-stage processing, including the allocation of arsenic (V) in the form of sediment saleable lithium, translation of arsenate lithium in arsenic acid using hydrochloric acid, recovery arsenic acid chloride of arsenic (III) using a combination of reducing agent - of sodium iodide and ascorbic acid or hydrazine.

The disadvantages of this technology:

- a multi-stage process,

-the use of expensive reagents (as of December 2012: the lithium chloride - from 650 RUB/kg sodium iodide - 1200 RUR/kg, ascorbic acid - 530 roubles/kg) in the process;

- additional pollution the resulting product - As 2 O 3 impurities compounds of lithium, iodine, the products of oxidation of ascorbic acid;

- similar to patent RU 2359725, technology EN 2389526 widely used highly toxic and volatile chloride, arsenic (III)included in List 2 of the precursors of chemical weapons, established by the Convention on the prohibition of chemical weapons.

Processing technology of arsenic-bearing raw materials in technical elemental arsenic

Technology described in PL 357396, describes the selection of elemental arsenic from water solution and includes the oxidation As(III) As(V) hydrogen peroxide, sedimentation arsenic in the form of low-solubility of calcium arsenate, obtaining of elemental arsenic effect on calcium arsenate phosphorous acid when heated to 80-150°C in large doses of 400% from quantity of calcium arsenate) phosphoric acid. The disadvantages of the method are a large number of process steps, the use of hazardous oxidizer when heated, the use of toxic compounds of phosphorous acid as reductant, a large number contaminated with arsenic waste in the process, the possibility of formation of toxic and explosive phosphine when heated phosphide acid reaction disproportionation:

Technology described in RF 2009276, formed the basis for the creation of industrial installations electrolysis launched into operation at the facility for destruction of chemical weapons in Gornyi Saratov region in 2009 After two years of operation it was decided to close the production due to the low output of arsenic and education 3-4 times the amount of arsenic wastes in the process of electrolysis.

Similar in content patents EN 2371391 and EN 2409687 describe the method of selection of elemental arsenic from solutions through reaction with reducing agents - thiourea dioxide in an alkaline environment.

Let us consider the processes occurring in water-alkaline solution of thiourea dioxide in the presence of ions

The excess of thiourea dioxide decomposes in the solution with the formation of urea, sulfur, elemental sulfur, sulfites and other sulfur compounds [4]. The resulting total reaction solution containing sulfur sodium, urea and residual quantities of arsenic (level 2 to 50 mg/l, which in 40-1000 times the applicable Macs for arsenic in natural water), finds no practical application and requires additional resources for recycling. The cheapest option disposal of such solution - natural or forced evaporation and burial mixtures of urea and inorganic salts on the landfill of waste (approximately, 3rd class of danger).

You can calculate some technological parameters of the application of such method for processing of APG in elemental arsenic:

-the average composition of the APG is 46.0% NaCl, Of 9.30% Na 3 AsO 4 , 44,1% Na 3 AsO 3 ;

- number of thiourea dioxide (DTM), required for the transfer of arsenic compounds in elemental arsenic, can be estimated, using the examples in patents: for EN 2409687 DTM is used as weight 2.16 g DTM/1 g As 3+ and 20 g DTM/1 g As 5+ ; for EN 2371391 applies a greater ratio of 4.8 g DTM/1 g As 3+ ;

-1 kg ANG contains on average 172,3 g As 3+ and 33.5 g As 5+ (calculation is made according to the formula

- weight of arsenic in the oxidation degree n+, m ANG - weight ANG, 1000 g, ω salt - mass fraction of this type of salt in raw materials, M(As) is molar mass of arsenic, 75 g/mol, M(salt) is molar the weight of this type of salt, 192 g/mol for Na 3 AsO 4 and 208 g/mol for Na AsO 3 4 ;

- the number DTM necessary for processing 1 kg ANG by way RU 2409687, is equal 172,3*2,16+33,53*20=1042,8 g;

- number of technological wastes per 1 kg ANG: from the reaction of the system (connection of arsenic-DTM) as a useful product displays only elemental arsenic. Therefore, the approximate number of dry waste (in case of 100% output for arsenic) will be equal to the sum of the masses feedstock and reductant, minus the mass of arsenic in raw materials: m-APPR =m ANG +m DTM-m As=1000+1042,8-(172,3+33,5)=1837,0 g waste, i.e. - 180% from the quantity of raw materials, which severely limits the possibility of using these methods.

In the patent RU 2396099 is another method of treatment of liquid reaction mass formed by alkaline hydrolysis of lewisite. The technology includes: a concentration of the reaction of the masses and the deposition of them oxide arsenic (III) acidification, as well as different options for processing tailings - cleaning formed when the concentration of sodium chloride from arsenic action DTM, processing of concentrate containing sulfur reducing agents for translation of arsenic (V) (III).

However, the method has many disadvantages: none of pathways is not brought to its logical conclusion. So, it is known that the processing DTM arsenic solutions cannot lead to the conclusion of arsenic from a solution. Best experimental results obtained for December 2012, give a value of 2 mg/l of residual concentration As in solution. In addition to incomplete clean, purified sodium chloride polluted large number decay products DTM - sodium sulfite and urea. For another tail - filtrate after deposition of oxide arsenic (III) also does not provide clear-handling mechanism, although it should contain a significant amount of arsenic - solubility oxide arsenic (III) in water at N.U. is about 2%.

In method EN 2412734 retrieves the element arsenic solutions containing sodium chloride effect of reducing agent is sodium borohydride, and then emptying salt solution on iron-containing sorbents. The method is quite expensive, as the restorer applied in prices for December 2012 is about 16,000 rubles/kg

Processing technology of arsenic-bearing raw materials into other products

Technology in the patent RF describes the acidification of sulphuric acid to pH 2.5 solution of raw materials and then add sodium sulphide with the formation of poorly soluble arsenic sulfide AS 2 S 3 .

The disadvantages of the technology are:

- creation of strongly acidic environment in the system containing products of decomposition of lewisite, can lead to the formation of chloride, arsenic (III) and lewisite, similar to the situation described in the discussion patent RU 2359725;

- allocation of uncontrolled quantities of hydrogen sulfide in the atmosphere;

- produced arsenic sulphide has a very small size of crystals, which leads to great difficulties in its filtering.

Technology in the patent RF includes adding to the solution of the raw material water solution hydrogen peroxide in the quantity ensuring the oxidation arsenite-ion battery

in arsenate

), evaporation of the reaction mass to the content of the arsenic ion in 120 g/kg, a cooling solution at pH>13 prior to vegetalization of sodium arsenate and separation last filtration.

However, this method has significant drawbacks: explosion hazard when working with hydrogen peroxide when heated, obtaining arsenic waste water after stage filtration, the limited application of sodium arsenate in the national economy, the lack of technical solutions for removal of polluted sodium chloride and other impurities.

Marketing researches show that of arsenic compounds the most widely used product in the national economy is the oxide arsenic (III), and lately there is a steady trend production and consumption of semiconductor compounds on the basis of gallium arsenide, raw material for which is high-purity arsenic [5].

After considering known hydrometallurgical processing technology of arsenic-bearing raw materials we can formulate the following requirements to technology processing ANG:

- possibility of processing into marketable products of arsenic (III) and (V)available raw materials;

- minimizing the number of technological waste;

- absence in the process of hazardous substances, such as chloride, arsenic, Arsin and other volatile hydrides non-metals, hydrazine;

- the minimum cost of reagents used in technology.

To meet these requirements found new technical solutions:

- the use of leaching, rather than dissolution ANG;

- the use of closed cycle "leach - preparation of a solution-the deposition of oxide arsenic (III) the return of the filtrate" exclusively for the production of oxide arsenic (III);

- use the module for processing solutions, unfit for further use in obtaining oxide arsenic (III).

The problem is solved two-step method:

1) Initially conducted grinding of raw materials to the granule size not more than 3 mm Prepared raw material is fed into the dipstick-dispenser solids. From the measuring device mounting raw material is served in a tank with a mixing device, which is the leaching of salts arsenic. For leaching system is used water - hydrochloric acid or the system filtrate - hydrochloric acid - water. The first system is used if there is currently no usable filtrate. The mass of water or filtrate is taken 1.4-1.6 times more mass of raw material. Hydrochloric acid is added to achieve a pH of the system in 9,5-10,5 that you want to translate arsenic salts in raw materials in digitoksina and digitoksina sodium, with the biggest solubility among sodium salts arsenic and arsenic acid [6]. The required amount of hydrochloric acid depends on the content of the total alkali in the party of raw materials and consistently within one party. Leaching is conducted 1-2 hours propaganda method, the tank should be equipped with a device for unloading of the suspension. Further suspension consisting of a salt solution and the solid phase, including sodium chloride (the main component), polluted by salts arsenic, insoluble organic compounds and bentonite, served on the strainer, which is filtering and washing sludge. The sediment is washed in the filter water for leaching of soluble salts arsenic. The method and amount of leaching depend on the technical design of this filter, as a rule, only two leaching, the total volume of which is equal to the volume of filtrate. Washed residue of sodium chloride after treatment according to the method (dissolution, filtering on thin filter, sorption treatment) meets the current standards on technical sodium chloride, and is suitable for preparation of solutions for killing of oil and gas wells and other purposes. The washing of the water combined with the filtrate and serves on the operation of the filter on the filter of thin clearing. For this operation is well suited filter press or other filter with large filtering surface. This activity is separated from the solution of fine sediment bentonite and insoluble organic matter. This sludge is sent for disposal by heat treatment. Leachate contains a mixture of dissolved salts: sodium chloride (close to saturated), digitoksina sodium, digitoksina sodium. Then, the solution is sent to the operation evaporation. Evaporation is carried out in evaporation apparatus in order to produce a concentrated solution of salts arsenic (III) (up of arsenic (III) above 10 g/100 g of water). Formed in the process of evaporation residue of sodium chloride is separated on the filter, washed and combined with sodium chloride obtained previously. The stage of evaporation of the effluent can be skipped in case the content of arsenic (III) in the raw materials is very high. Evaporating device must be equipped with a device for unloading of the suspension. After the precipitate of sodium chloride is Pasadena oxide arsenic (III) of one stripped off solution by adding hydrochloric acid to pH 6-7. Suspension containing oxide arsenic, filtered, oxide arsenic washed small amount of water, which is combined with the filtrate. The precipitate containing 80% mass and more oxide arsenic (III), and water, and the admixture of sodium chloride, dried on filter and sent to the technical oxide arsenic (III) the method of sublimation purification by known technologies. The filtrate is obtained after the separation of oxide arsenic (III), dispatched in the beginning of the process to conduct leaching of salts arsenic from the new consignment of raw materials. The filtrate is rich in sodium chloride and oxide arsenic (III), which provides its permanent staff except the content of salts arsenic (V)that do not appear in large numbers from the solution by the above operations.

To sum up, the first stage includes cyclic repetition of successive stages:

- leaching of salts arsenic from raw material with formation of heterogeneous systems;

- separation of heterogeneous systems on a solid phase and the working solution;

- concentration of the working solution and separation of concentrated solution from the formed sediment;

- deposition of oxide arsenic (III) by acidification of the working solution and separation of sludge oxide arsenic (III) by filtration;

- the return of the filtrate to the first stage of the process.

2) the Second stage of the technology applied in the case of batches of arsenic (V). It is that after repeat cycle of operations of the first phase of 3 to 10 times the operation of removing arsenic (V) of the working solution by restoring them to their compounds arsenic (III), or to elemental arsenic.

The first stage of processing technology ANG responsible task of translating salts arsenic (III)contained in the raw materials, oxides of arsenic (III), however in raw materials also contains salts arsenic (V), whose concentration in the working solution at each subsequent cycle increases. This leads to contamination of precipitation sodium chloride significant amount of salts arsenic (V)that may negatively affect the entire technology. For this reason, should conduct periodic conclusion of arsenic (V) of the operating cycle. Frequency output arsenic (V) of the operating cycle depends on the content of sodium arsenate in the feedstock, is the optimal value of 1 transaction for every 3 cycle of the first stage of the process up to 1 operation for every 10 cycles. The excretion of arsenic (V) of the solution should be done with the content As(V) in the solution at the level of 10 g/100 g of water. Concentration As(V) in solution with each new cycle increases linearly (loss compounds As(V)that enter in the composition of precipitation is insignificant at concentrations As(V) less than 10 g/100 g of water), so the number of cycles of the first phase, after which it should be output As(V) of the solution, can be estimated by the decision of the empirical equations

- mass fraction of sodium arsenate in the party ANG, n - the given number of cycles.

For the conclusion of a working solution of arsenic (V) can be used to restore arsenic (III), or the recovery of elemental arsenic. As a restore operation, arsenic (V) result in the contamination of the solution by decay products of reducing agent, then use the resulting solution in the cycle of the first stage it is impossible, instead of the solution removes residual amounts of arsenic and the solution is sent for recycling. For translation of arsenic (V) arsenic (III) can be used by any of the known reducing agents medium strength, for example, sodium sulfite. The reaction is carried out in slightly Wednesday, after which the pH is increased up to 6-7, is the Department of oxide arsenic (III), and the filtrate is sent for recycling.

Another option procedures of the second stage is the conclusion of arsenic (V) of the solution by means of thiourea dioxide. In this case, a solution containing a significant amount of salts arsenic (V), served in a tank with mixing device is warmed up to 60/80, podslushivaet to pH 10-10,5 by adding the estimated quantity of solid sodium hydroxide (about 4 g per 1 g of arsenic(V) in solution. Further to the solution batch add restorer - thiourea dioxide in the amount corresponding to the stoichiometric ratio plus the excess of 20% (4,32 g thiourea dioxide per 1 g of arsenic (V) in the solution). Formed in the reaction of elemental arsenic filtered, dried in an inert atmosphere and sent to the operation of sublimation purification or oxidizing roasting of obtaining oxide arsenic (III) by known technologies. In this case the process of removal from circulation of arsenic (V) leads to contamination resulting mortar admixtures of sodium sulfite and urea, so after carrying out such operations, and precipitate elemental arsenic should send the filtrate for recycling. For the disposal of the filtrate is evaporated and the dry mixture of salts containing sodium chloride, sodium sulfite and carbamide, as well as arsenic compounds at 40 mg/kg of waste is sent for disposal at the landfill of waste. The amount of generated waste can be estimated in the following examples:

Example 1.

The content As(V) in raw materials 14,5%mass, the content of bentonite and insoluble organic substances - 4 wt.%, disposable loading of raw materials 5 kg of the 3rd cycle of the first stage with the subsequent conclusion of arsenic (V) use of thiourea dioxide:

NaCl 1,886 kg

Total: 15,259 kg

Total: 15,259 kg

Entrance

9. Filtering and washing As

8.3 Suspension

9.2 the Filtrate

As 0,834 kg

As 0,833 kg

Na 2 SO 3 3,354 kg

H 2 O 1,0 kg

(NH 2 ) 2 CO 1,597 kg

NaCl 1,886 kg

9.3 Sediment

H 2 O 7,588 kg

Na 2 SO 3 3,354 kg

(NH 2 ) 2 CO 1,597 kg

9.1 Wash water - 1,0 kg

NaCl 1,886 kg

H 2 O 7,588 kg

Total: 16,259 kg

Total: 16,259 kg

Entrance

10. The evaporation of the effluent

9.2 the Filtrate

10.1 Sediment-6,837 kg

Na 2 SO 3 3,354 kg

Na 2 SO 3 3,354 kg

(NH 2 ) 2 CO 1,597 kg

(NH 2 ) 2 CO 1,597 kg

NaCl 1,886 kg

NaCl 1,886 kg

H 2 O 7,588 kg

10.2 Water - 7,588 kg

Total: 14,425 kg

Total: 14,425 kg

The total amount of waste - 15*4%+6,837=7,437 kg 15 kg of processed raw material, which is 49,6% by weight of raw materials.

Example 2.

For raw materials with a lower content As(V) the processing of reducing agent frequently required, suspension A.1.4 corresponds to 10-th cycle of the first stage of processing of raw material with the content As(V) 4.3% (mass). In this case, if the total content of bentonite and RGR is 4 wt.% and as a reducing agent used DTM, the total amount of waste by 50 kg of processed raw material is equal to 50*4%+6,837=8,837 kg, representing 17.7% of the mass of raw material.

Examples show that this method two-stage processing of the raw material suitable for processing into marketable products of arsenic (III) and (V)contained in ANG, and can significantly reduce oktobrova - from 180% for reducing technology EN 2409687 to 17.7% - 49.6% and to reduce the consumption of reducing agents in 5 and more times in depending on the composition of the feedstock. Also it is visible, that at the first stage of the process as a reagent used exclusively hydrochloric acid, which provides low cost processing.

Literature

[1] report on the implementation integral part of works for state needs, on theme "Scientific andtechnical support operational work on the facility for destruction of chemical weapons in pogonny Saratov region", name integral part of "Operation production, auxiliary buildings and facilities and provision of activities related to processing of the reaction mass and industrial waste generated as a result of destruction of chemical weapons at the facility", Saratov, 2009

[2] URL: http://www.opcw.org/ru/konvencija-o-khimicheskom-oruzhii/prilozhenie-po-khimikatam/v-spiski-khimikatov/ from 05.12.2012,

[3] V.N. Alexandrov., V.I. Emelyanov Toxic substances, Ed. G.A. Sokolsky. - 2-e Izd. - M: Voenizdat, 1990. - 272 S.

[4] Budanov V.V. Makarov, ST. Chemistry of sulfur-containing reducing agents: (Rongalit, dithionite, thiourea dioxide). M: Chemistry 1994. - 139 C.

[5] Marketing research of the markets of consumption of arsenic-bearing marketable products. Final report on research. Cipher "Products - M". GNIICHTEOS, 2005.

[6] Kaminsky UD, Kopylov NI Arsenic. Novosibirsk: Siberian University publishing house, 2004, 368 S.

1. Method for processing of technical arsenite sodium hydrolysis into marketable products, including a cyclic repetition successive stages: - leaching of salts arsenic from raw materials with a solution of hydrochloric acid added to pH 9,5-10,5, with the formation of heterogeneous systems; - separation of heterogeneous systems on a solid phase and the working solution; - the concentration of a working solution by evaporation to arsenic (III) above 10 g/100 g of water and the separation of concentrated working solution from the formed sediment; - deposition of oxide arsenic (III) by acidification of the working solution and separation of sludge oxide arsenic (III) by filtration; - return of the filtrate to the first stage of the process.

2. The method according to claim 1, characterized in that, after the repeat loop of the above operations from 3 to 10 times the operation of removing arsenic (V) of the working solution by restoring them to their compounds arsenic (III), or to elemental arsenic.